It is known in the art that the activity of Co-Mo-Al.sub.2 O.sub.3 and Ni-Mo-Al.sub.2 O.sub.3 catalysts for denitrogenation and to a lesser extent for desulfurization of mineral oils can be improved by incorporating minor proportions of silica therein, usually as a cogel with the alumina component. Conventionally, an alumina-silica cogel base is prepared by coprecipitation, followed by washing, adding alumina hydrogel, drying, pelleting or extruding, and calcining. The calcined base is then impregnated with an aqueous solution of solutions of Co and/or Ni salts, and with a soluble Mo compound such as ammonium heptamolybdate. The impregnated base is then again dried and calcined. This procedure is expensive, involving at least two calcinations, and moreover does not provide an optimum distribution and retention of active sites, particularly the Ni and/or Co hydrogenation sites and the protonic acidity sites generated by the alumina-silica cogel.
I have now discovered a novel catalyst preparation procedure which involves only a single calcination step, and which yields silica-promoted catalysts of unexpectedly high activity, considering the unorthodox nature of the process. In brief summary the process involves the following essential steps:
(A) ion exchanging a silica-alumina hydrogel containing about 50-90 wt.% SiO.sub.2 with an ammoniacal solution of a cobalt and/or nickel compound to incorporate therein by ion exchange at least about 2 wt.% of Co and/or Ni; PA1 (B) homogeneously admixing the undried solid product from step (A) with a boehmite alumina hydrogel, a molybdenum compound, in most cases additional cobalt and/or nickel compound, and sufficient water to provide an extrudable plastic mixture; PA1 (C) extruding the plastic mixture to provide extrudates of desired size and shape; and PA1 (D) drying and calcining the extrudates.
The principal unorthodox feature of this process is that most or all of the Ni and/or Co component is selectively combined with the silica-alumina hydrogel portion of the support, while the molybdenum component is selectively combined with the boehmite alumina portion. Traditional thinking in the art is that the active metals should be distributed homogeneously on all portions or components of the support material. I have discovered however that if the respective active metal components are selectively deposited upon colloidal size micelles of the respective support components, such as the micelles making up silica-alumina hydrogels and boehmite hydrogels, a sufficiently intimate association of all active metal components is obtained upon drying, calcining and sulfiding the catalysts. Such a result would not be obtainable if either of the support components were in the form of a dried or calcined gel when contacted with the active metal component, for as is well known the drying or calcining of such hydrogels brings about a coagulation of the colloidal micelles into much larger aggregates. Any active metal penetrating into the interior of such aggregates obviously could not form an active association with a different active metal deposited on other support particles. But in the case of colloidal micelles, most of the active metal is deposited on the exterior surfaces thereof, and thus is available for intimate contact with active metals deposited on the exterior surfaces of other micelles.